In the formation of integrated circuits (ICs) it is often necessary to deposit thin material layers or films, such as films containing metal and metalloid elements, upon the surface of a substrate, such as a semiconductor wafer. One purpose of such thin films is to provide conductive and ohmic contacts for the ICs and to yield conductive or barrier layers between the various devices of an IC. For example, a desired film might be applied to the exposed surface of a contact or via hole in an insulating layer of a substrate, with the film passing through the insulating layer to provide plugs of conductive material for the purpose of making electrical connections across the insulating layer.
One well known process for depositing such films is chemical vapor deposition (CVD), in which a film is deposited on a substrate using chemical reactions between various constituent or reactant gases, referred to generally as process gases. In CVD, process gases are pumped into the process space of a reaction chamber containing a substrate. The gases react in the process space proximate a surface of the substrate, resulting in the deposition of a film of one or more reaction by-products on the surface. Other reaction by-products that do not contribute to the desired film on the exposed substrate surfaces are then pumped away or purged by a vacuum system coupled to the reaction chamber.
One variation of the CVD process, which is also widely utilized in IC fabrication, is a plasma-enhanced CVD process or PECVD process in which one or more of the process gases is ionized in a gas plasma to provide energy to the reaction process. PECVD is desirable for lowering the processing temperatures and the amount of thermal energy that are usually necessary for a proper reaction with standard CVD. In PECVD, electrical energy is delivered to the process gas or gases to form and sustain the plasma, and therefore, less thermal energy is needed for the reaction.
Another well-known IC fabrication process is sputter deposition, which also utilizes an ionized plasma, but relies upon physical deposition rather than a chemical reaction. Sputter deposition is therefore referred to as a physical vapor deposition or PVD process. PVD processes utilize ionized particles of a charged gas plasma to bombard a target of material and dislodge or "sputter" away material particles from the surface of the target. The material particles then deposit on the substrate which is positioned in the processing chamber proximate the target. In sputter deposition, a plasma gas is introduced into a processing chamber under vacuum. The target to be sputtered is supported on an electrically biased base within the processing chamber whereon the target develops an electrical charge or bias. The power supply which sustains the electrical charge on the target also couples electrical energy into the plasma. The electrical energy ionizes the gas particles to form the plasma of ionized particles, and the ionized particles are attracted to the biased target surface, bombarding the surface and sputtering the material particles from the target. The particles of target material then deposit on the substrate to form a material layer.
Material layers will be deposited by both PVD and CVD methods on a single substrate during IC fabrication. Therefore, in the industry, it has become very common to incorporate a PVD process chamber and a CVD process chamber together within a single processing system, along with a variety of other process chambers. In this way, the substrates to be processed may be transferred quickly and efficiently between the various chambers. Such multi-chamber systems are often referred to as cluster tools because they include a cluster of different processing chambers or modules which are utilized together. Such cluster tools also include a common transfer chamber or module which is operable for transferring the various substrates between the various process chambers in a controlled manufacturing sequence. The transfer chamber will usually incorporate a substrate transport device or substrate handler to move substrates back and forth between the transfer chamber and the various processing chambers coupled to the transfer chamber.
While cluster tools have provided an efficient and cost effective means for IC fabrication, they have had some inherent drawbacks. Specifically, the process gases and process by-products from one chamber can migrate to other chambers, where they may act as contaminants to the processes performed in those other chambers. For example, process gases and by-products within a CVD chamber will tend to migrate into a PVD chamber through the common cluster tool transfer chamber when the substrates are moved between the various process chambers. The CVD gases which are commonly used for IC fabrication actually act as contaminants within the PVD chamber and degrade the quality of the PVD films by being trapped within the films or by exposing the deposited films to undesired chemical attack. HCl is a common by-product of some CVD processes, and will have a corrosive effect on a PVD aluminum film, for example.
To integrate PVD and CVD chambers into a single cluster tool, it is necessary to reduce and minimize the flow of residual CVD contaminants from the CVD process chamber to the PVD chamber. One possible solution utilizes a high vacuum or turbomolecular pump which is connected to the CVD process chamber for purging the chamber to a pressure of around 10.sup.-6 Torr prior to transferring the substrate out of the CVD chamber. Coupling the turbomolecular pump to the CVD chamber, however, not only increases the cost of the processing tool, but also complicates its construction and maintenance.
Another proposed solution for preventing contaminants is to purge the CVD chamber with one or more reactive gases which render the by-products and residual process gases more volatile so that they may be more readily removed from the vacuum system. However, the additional step of exposing the CVD chamber to the reactive gases slows down the throughput of the processing system and thus increases the overall cost of IC fabrication.
Yet another solution might be to use a separately standing chamber between a process chamber and a system transfer chamber to isolate the process chamber from the transfer chamber. For example, an additional chamber might be positioned in-line horizontally between the transfer chamber and process chamber with isolation valves at either end. The isolation valves will selectively isolate the center chamber from the process chamber and transfer chamber. While isolation could therefore be provided without introducing another reactive gas or installing an expensive turbomolecular pump, such a solution still has several drawbacks. First, the horizontal linear arrangement required will increase the footprint of the processing system. Furthermore, the two isolation valves of the center chamber cannot be opened simultaneously without providing a direct path between the process chamber and transfer chamber and thus severely impairing the function of the center chamber. Therefore, an extra substrate handler will be required within each buffer chamber on the cluster tool, in addition to the substrate handler already existing in the transfer chamber, so that the substrate may be moved to the process chamber when the center chamber is isolated from the transfer chamber. Not only does the extra hardware of an additional substrate handler increase the cost of the cluster tool, but it also reduces its reliability while increasing the complexity of operating the various modules within a controlled manufacturing sequence.
Accordingly, it is an objective of the present invention to reduce contaminants migrating between different chambers of a cluster tool, and specifically to reduce the migration of process and by-product gases from a CVD chamber to a PVD chamber within such a cluster tool.
It is another objective of the present invention to achieve such contaminant reduction between multiple process chambers while maintaining an efficient manufacturing throughput for the processing system.
It is still another objective of the invention to reduce contamination without increasing the overall cost, size, and complexity of the cluster tool.
These objectives and other objectives are further discussed hereinbelow and are addressed by the present invention.